In portable handheld work apparatus such as a motor-driven chain saw, a safety braking device is provided which brings the running saw chain to standstill in response to a kickback of the chain saw. The saw chain is driven by a drive sprocket which is connected to a clutch drum of a centrifugal clutch so as to rotate therewith. A brake band wraps around the clutch drum and is actuated by a brake lever. The brake band is loaded with force when it is in its braking position (contact engagement with the clutch drum). The braking device is transferred into the braking position by a trigger (hand guard) in response to a kickback.
U.S. Pat. No. 4,683,660 discloses a braking arrangement wherein the saw chain is connected to the throttle lever in the handle via the pull wire of a Bowden cable. The throttle flap of the carburetor of the engine is actuated via a second actuator acting on the throttle lever in order to control the engine speed. When the throttle lever is pivoted, the braking device is first released via the first actuator (pull wire of the Bowden cable) and the throttle flap is simultaneously opened via the second actuator so that the engine speed can increase. When the throttle lever is released, the braking device is again transferred into the braking position so that a running down of the work tool, namely the saw chain, is prevented.
Since the braking device is simultaneously released with the throttle lever and the engine speed is increased, it cannot be precluded that the centrifugal clutch is engaged when the brake band is still partially in contact engagement with the clutch drum thereby causing the clutch drum to be driven. On the one hand, this causes a high wear of the braking device and leads on the other hand, to a considerable thermal load on the centrifugal clutch which can have consequences for the operation thereof. If the braking torque developed by the partially applied brake band is greater than the drive torque, then the centrifugal clutch will slip whereby its wear will be increased and a thermal overload cannot be precluded. When releasing the throttle lever, the braking device is applied immediately and the throttle flap is closed. The speed of the engine is first higher than the idle speed because of the inertia of the moved masses and it is for this reason that the above described conditions can occur even when releasing the throttle lever.